This application claims benefit of Japanese Application No. Hei 11-254447 filed in Japan on Sep. 8, 1999, the contents of which are incorporated by this reference.
The present invention relates to an endoscope system and, more particularly, to an image pickup optical system of an endoscope system that uses a solid-state image pickup device.
Video endoscopes using a small-sized CCD (solid-state image pickup device) have been prevailing in recent years. This type of endoscopes allows a large number of people to observe the morbid part in the patient""s body cavity on a monitor and hence permits the patient to be examined and diagnosed by a plurality of doctors. It is also possible for the patient to be diagnosed while observing his/her own morbid part. This is a great advantage. Among these endoscopes, so-called magnifying endoscopes have attracted special interest recently. The magnifying endoscopes allow close-up observation of the morbid part to inspect the micro-structure thereof, thereby deciding the degree of infiltration in a small morbid region and determining an extent to which the morbid part should be excised. This type of magnifying endoscope requires focus control to be effected according to the object distance differing for different observation modes, i.e. normal observation and close-up observation.
For example, Japanes Patent Post-Exam Publication Number (hereinafter referred to as xe2x80x9cJPxe2x80x9d) Hei 6-3495 discloses a mechanical arrangement for moving a solid-state image pickup device without moving an optical system to effect focus control. The disclosed arrangement allows realization of an endoscope having a smaller amount of decentration of lenses than in the case of an arrangement in which focus control is effected by moving the optical system.
Meanwhile, simultaneous solid-state image pickup devices in which color filters are placed in front of light-receiving parts have been remarkably reduced in both size and cost: in recent years. Accordingly, the application of such a simultaneous solid-state image pickup device to an endoscopes is under consideration. FIG. 8 is a conceptual view of the cross-section of a simultaneous solid-state image pickup device. Light-receiving parts 1 are photodiodes for converting light into electric signals. A color filter 2 is placed in correspondence to each light-receiving part 1. Normally, a light beam 3 passes through a color filter 2 corresponding to each pixel, and an electric signal outputted from each light-receiving part 1 is computed in an electric signal processing circuit (not shown) to generate a color signal.
However, when there is a light beam 4 entering at a large incident angle, light passing through a color filter 2 corresponding to a light-receiving part 1 next to the light-receiving part 1 for the light beam 3 enters the latter light-receiving part 1. As a result, an unwanted electric signal is outputted correspondingly. If this signal is subjected to normal signal processing, color reproduction different from normal results undesirably. In the ordinary optical system, the incident angle increases as the image height increases. Therefore, the periphery of the image is undesirably colored owing to the above-described phenomenon. This problem is known as xe2x80x9ccolor shadingxe2x80x9d.
FIG. 9 is a conceptual view showing the image position 5xe2x80x2 and the solid-state image pickup device position 5 when the optical system of JP Hei 6-3495 is focused on a far object and also showing the image position 6xe2x80x2 and the solid-state image pickup device position 6 when the optical system is focused on a near object. In the figure, the angle at which a light ray 8 from the near object is incident on the solid-state image pickup device varies to a considerable extent with respect to a light ray 7 from the far object because of the change in position of the solid-state image pickup device. This gives rise to the problem of color shading phenomenon accompanying focus control when a simultaneous solid-state image pickup device is used. JP Hei 6-3495 is an invention relating to a method of simply moving a solid-state image pickup device and makes no mention of the problem of color shading phenomenon.
Endoscopes are wide-angle optical systems having a field angle ranging from 120xc2x0 to 140xc2x0. Therefore, it is likely that rays outside the field angle will collect at a position near the effective image pickup area to produce flare. FIG. 10 is a diagram showing a light ray 9 at the maximum field angle in a wide-angle optical system, together with light 10 outside the field. Light 10 outside the field emanates from an object outside the field, e.g. a bright spot in a mucous membrane of high reflectance, and forms an image at a position extremely close to the image-formation position of the light ray 9 at the maximum field angle. If a structural member or portion of a solid-state image pickup device 11 is present at the image-formation position of the light 10, it serves as a secondary light source, and scattered light is generated therefrom and enters an effective area 13 as unwanted light 12. Therefore, it is necessary to place a flare stop 14 so as to prevent the light 10 from entering the solid-state image pickup device 11.
However, in the arrangement wherein the solid-state image pickup device moves, the incident angle of rays on the solid-state image pickup device varies, as shown in FIG. 9. Consequently, the ray height at the position of the flare stop 14 varies. Therefore, flare cannot satisfactorily be prevented because of focus control. JP Hei 6-3495 makes no mention of the problem how to prevent the occurrence of flare.
Further, in JP Hei 6-3495, the solid-state image pickup device is moved by using the outer periphery thereof as a guide. In general, however, there are variations in the displacement of the outer periphery of the solid-state image pickup device with respect to the center of the image pickup area. That is, the invention disclosed in JP Hei 6-3495 involves the problem that the center of the field is displaced from the center axis of the endoscope. If the solid-state image pickup device is arranged so that the center of the outer periphery of the image pickup device and the center of the image pickup area are made,coincident with each other in order to prevent the occurrence of the above-described problem, the solid-state image pickup device becomes costly.
In view of the above-described problems with the prior art, an object of the present invention is to provide an image pickup optical system for use in a magnifying endoscope or the like in which focus control is effected by moving a solid-state image pickup device, which is designed so that when a simultaneous solid-state image pickup device is used, color shading is suppressed satisfactorily, and flare-preventing measures are taken satisfactorily, and further the center of the field and the center axis of the endoscope are allowed to coincide with each other at a reduced cost.
According to a first aspect thereof, the present invention provides an image pickup optical system that is an optical system capable of being focused at a plurality of object distances by moving a solid-state image pickup device. The optical system includes an optical element by which light rays whose incident angle varies in accordance with focus control are made telecentric.
FIG. 1 is a conceptual view for describing the image pickup optical system according to the present invention. A pupil-varying field lens 15 is placed on the object side of a movable solid-state image pickup device 11 to make light rays entering the solid-state image pickup device 11 close to being telecentric. Consequently, a light ray 7 from a far object and a light ray 8 from a near object can be passed through a color filter corresponding to each light-receiving part.
Thus, it is possible to prevent color shading irrespective of the movement of the solid-state image pickup device.
Further; it is desirable for the present invention to satisfy the following condition:
xcex8 less than tanxe2x88x921(p/2L)xe2x80x83xe2x80x83(1)
where xcex8 is the maximum incident angle of rays, inclusive of the angle subtended by rays that depends on the F-number; p is the pixel pitch; and L is the distance to the farthest color filter from the light-receiving part.
Thus, it is possible to define the incident angle and the angle subtended by rays so that a light ray will not pass through a color filter corresponding to a neighboring pixel. The color shading gives rise to a problem particularly in a CCD of high pixel density. The reason for this is that it is necessary to reduce the angle of oblique incidence because the pixel pitch p in the condition (1) is small.
One of the objects of the present invention is to provide a magnifying endoscope, and the purpose of observing an enlarged image of the morbid part is to inspect the micro-structure thereof. Therefore, increasing the number of pixels of the solid-state image pickup device is very effective. Accordingly, the present invention is particularly effective when combined with a solid-state image pickup device of high pixel density.
It is desirable that the field lens in the present invention should be stationary with respect to the movable solid-state image pickup device. If the field lens is stationary, when focus control is effected by moving the solid-state image pickup device, the variation in the field angle can be minimized. Therefore, it is possible to obtain an image that gives no sense of incongruity to the observer, advantageously.
If a reflecting prism is placed immediately in front of the solid-state image pickup device, the diameter of the endoscope can be reduced favorably even when the size of the image pickup area is large. However, because of the angle dependence of the reflectance of the prism, if the variation in the incident angle is large, the brightness at the periphery of the image field reduces unfavorably. Stabilization of the incident angle of rays by use of a field lens is also effective in solving this problem.
Thus, according to the first aspect of the present invention, a field lens is arranged to act positively in a magnifying endoscope of the type in which a solid-state image pickup device is moved, thereby providing advantageous effects as well as effectively preventing color shading.
According to a second aspect of the present invention, the above-described image pickup optical system has a device for preventing entrance of unwanted light that is placed on the object side of the solid-state image pickup device.
Placing the unwanted light entrance preventing device on the object side of the solid-state image pickup device is effective from the viewpoint of flare prevention. As has already been stated above, the change of the ray height at the maximum field angle caused by the movement of the solid-state image pickup device is minimized by the action of the field lens in the arrangement according to the first aspect of the present invention. In this arrangement, if a flare stop is placed at a position where the change of the ray height is minimal, unwanted light can be cut off efficiently even if focus control is effected. It is desirable that the position where the change of the ray height is minimal should be in the vicinity of the field lens, more desirably immediately behind the field lens. When a reflecting prism is present between the field lens and the solid-state image pickup device, blocking extra-field light before it enters the reflecting prism is particularly effective in preventing the occurrence of flare.
According to a third aspect thereof, the present invention provides an optical system capable of being focused at a plurality of object distances by moving a solid-state image pickup device. The solid-state image pickup device includes at least an optically positioned optical element. The movement of the solid-state image pickup device is effected by sliding it using the outer peripheral portion of the optical element as reference.
Originally, the solid-state image pickup device has a large displacement between the center of the outer periphery thereof and the center of the effective area thereof. Therefore, to position the solid-state image pickup device accurately, it is preferable to cement a positioning optical element (e.g. a plane-parallel plate) to the front of the solid-state image pickup device and to slide the solid-state image pickup device by using the positioning optical element as a guide after making an adjustment such that the center of the outer periphery of the optical element is coincident with the center of the effective area of the solid-state image pickup device. By doing so, it is possible to prevent the center of the field from being displaced from the center axis of the endoscope even when the solid-state image pickup device is slid, without making the solid-state image pickup device costly.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.